There are birth control methods that are based on the observation of cervical mucus precisely because it varies greatly in each phase of the cycle.
At the end of the menstrual period, the cervical mucus is thin and clear (or whitish). Normally in the ovulatory phase (in the days immediately before ovulation and one day after ovulation) its characteristics change to facilitate the entry of sperm into the uterus and fertilization of the egg.
On those days it is usually whiter and above all it changes consistency, it is less dense and more filamentous. This can help to identify the most fertile days (which are those preceding ovulation up to 24 hours after it has taken place).